1. Field of the Invention:
This invention relates to electrostatic spraying of active materials onto surfaces and is particularly related to low volume applications.
2. Discussion of the Background:
Low-volume electrostatic spraying has been used from time to time in agriculture to spray pesticides on crops. Additionally, electrostatic coating operations are routinely used industrially to coat an active material, such as a paint or a doping agent, on a surface. In general, electrostatic coating of surfaces involves forming the coating material into finally divided particles or droplets, charging the particles or droplets to a single polarity (e.g., negative) and the surface to be coated to a different polarity (e.g., positive). Even at the ground potential (E=0), the coating target surface has induced into it from the "ground reservoir" a very appreciable net charge of sign opposite to the incoming charge cloud of coating particles. As a result of electrostatic attraction and the proximity of the particles or droplets to the surface to be coated, electrostatic forces move the particles toward the surface where they are deposited to form a coat or layer. Various prior art electrostatic coating applications are more sophisticated modifications of this simple situation. They differ from one another in the manner in which the particles are formed, the means by which they are charged, the particular aspects of the methods by which the particles are distributed about the surface, and the manner in which the particles collect on the surface. A review of prior art electrostatic processes can be found in Moore, A. D., Ed., Electrostatics and its Applications, Wiley and Sons, 1973, particularly pages 250-280.
The use of electrostatic spraying or coating is wide spread in controlled industrial environments where low-volume painting and other coating operations are generally conducted. Electrical hazards due to high voltages that are typically used are minimized in such controlled industrial environments. However, electrostatic spraying has recently been made available to agricultural spraying operations through the use of electrostatic spray nozzle systems using shielded voltage sources. Examples of agriculture applications are set forth in numerous United States patents, of which the following are examples.
Law, U.S. Pat. No. 4,004,733, discloses an electrostatic spray nozzle system using a low voltage power supply that is particularly designed for agriculture applications. Law, U.S. Pat. No. 4,168,327 describes a method for controlling the space-charge density of the spraying operation in order to produce an optimal deposition level. Malcolm, U.S. Pat. No. 4,328,940, describes a method of spraying electrically charged particulate materials to ground from a low flying airplane. The method includes the steps of imposing an electrical charge of a given polarity upon an emitted primary spray of particulate material while concurrently emitting a secondary spray of oppositely charged particulate material to avoid production of an opposite charge on the aircraft during the spraying operation. DeVittorio, U.S. Pat. No. 4,341,347, discloses a nozzle capable of imposing a high electrostatic charge on very small particles in order to improve their spray characteristics. All of these patents disclose various low volume spraying operations.
Nevertheless, their remains a need for methods of electrostatic spraying which utilize even lower volumes of liquids and which produce less waste than is now obtainable with current methods.
SUMMARY OF THE INVENTION
Accordingly, one obejct of this invention is to provide a novel method for depositing particulate matter on a surface having an increased efficiency of deposition, thereby reducing the volume of the coating material used and eliminating waste or harmful environmental effects.
This and other objects of the invention as will hereinafter become more readily apparent have been accomplished by providing a method for increasing the efficiency of deposition of particulate matter on a surface, which comprises preparing a dispersion of electrostatically charged particles of an active material to be deposited on the surface, wherein the particles of active material have an average radius of r.sub.pl and a total volume of V.sub.L1, in a space adjacent the surface which contains a dispersion of particles of an inert material electrostatically charged with the same polarity as the particles of active material, wherein the particles of inert material have an average radius of r.sub.p2 and a total volume of V.sub.L2, wherein r.sub.p1 is greater than r.sub.p2 and ##STR2## This method is particularly useful in agricultural applications but is also useful in any other process (such as spray painting) in which a particulate matter is being deposited on the surface.